Cancer is a leading cause of mortality in industrialized countries. Many chemotherapeutic agents have been developed over the past 50 years for the purpose of cancer treatment. Majority of the chemotherapeutic agents can be classified into alkylating agents, antimetabolites, anthracyclines, plant alkaloids, topoisomerase inhibitors, and antitumor agents. All of these pharmaceutical agents affect cell division or DNA synthesis and bring about therapeutic effects through a mechanism that functions in some way.
Effectiveness of a particular chemotherapeutic agent is different among cancers or patients, or is different depending on the time course in individual patients. Cancer cells exposed to chemotherapeutic agents develop resistance to these chemotherapeutic agents, and similarly often develop cross-resistance to a plurality of other anticancer agents. Furthermore, to control the side effects resulting from cell damage by these chemotherapeutic agents on normal cells through the above-mentioned mechanism of these agents, the dosage or usage of the agents is often restricted.
Instead of conventional chemotherapeutic agents, molecularly-targeted drugs which target molecules expressed specifically on cancer cells are being developed recently. With the appearance of these molecularly-targeted drugs, side effects intrinsic to conventional chemotherapeutic agents can be avoided, and cancer treatments that contribute to the QOL of cancer patients are becoming feasible. Such molecularly-targeted drugs include small-molecule pharmaceutical agents as well as high-molecular-weight pharmaceutical agents such as antibodies. Therapeutic antibodies are molecules that are inherently present in the body, and have the advantage of low toxicity on living organisms, as well as the advantage of exhibiting therapeutic effects by specifically damaging target cells by an action mechanism other than the mechanism of small-molecule pharmaceutical agents, such as cytotoxic activity mediated by effector functions. Accordingly, many therapeutic antibodies have been recently placed on the market
Therapeutic antibodies targeting Epiregulin, which is highly expressed in colon cancer, lung adenocarcinoma, pancreatic cancer, stomach cancer, and kidney cancer, have been disclosed as antibodies that specifically damage target cells by an action mechanism other than the mechanism of small-molecule pharmaceutical agents, such as cytotoxic activity mediated by such effector functions (Patent Document 1). Specifically, measurement of complement-dependent cytotoxicity (CDC) activity and antibody-dependent cell-mediated cytotoxicity (ADCC) activity of anti-Epiregulin antibodies revealed that anti-Epiregulin antibodies have CDC activity and ADCC activity on Epiregulin-expressing cells. Furthermore, anti-Epiregulin antibodies were found to have proliferation inhibitory effects on cancer cell lines through neutralizing action. Furthermore, from the above-mentioned findings, anti-Epiregulin antibodies were revealed to be effective for diagnosis, prevention, and treatment of various primary and metastatic cancers.
Any novel candidate pharmaceutical agent including anticancer agents such as those described above must pass strict trials to become commercially available. For example, these trials are classified into preclinical trials and clinical trials. Generally, the latter is further categorized into phase I trial, phase II trial, and phase III trial, and is performed on human patients, whereas the former studies are performed using animals. Generally, an objective of preclinical studies is to demonstrate that the drug candidate is potent as well as effective and safe. Specifically, the objectives of these animal studies are to demonstrate that the pharmaceutical agent is not carcinogenic, mutagenic, or teratogenic, as well as to understand the pharmacokinetics of the pharmaceutical agent. Clinical studies on administration of a test pharmaceutical agent to humans are permitted only when the safety and efficacy of the test pharmaceutical agent towards animals are established in preclinical studies.
In many cases, the action of a small-molecule test pharmaceutical agent (for example, a novel anticancer agent derived from anthracycline) in animals may become an indicator for anticipated actions of the pharmaceutical agent when administered to humans. Therefore, generally data obtained from such preclinical studies may be highly predictable of actions that will take place when it is administered to humans. However, such predictability is not obtained in every type of test pharmaceutical agent; and predictability from results of preclinical studies, and the possibility that candidate pharmaceutical agents are approved in clinical studies drop considerably.
Generally, antibodies can function through highly specific recognition of target molecules which are typically proteinaceous. In most cases, test antibody pharmaceutical agents are monoclonal antibodies, and recognize only a single site or a single epitope on a target molecule. Since monoclonal antibodies conventionally have a high target-identifying function, antibodies have become candidates of great interest for development of pharmaceutical agents, but on the other hand, this identifying function makes preclinical studies difficult in some cases. This is because there are species-specific variations in the target molecule sequences bound by these antibodies. For example, a monoclonal antibody that specifically recognizes molecule Y via epitope X in humans and binds to this molecule will be tested for the corresponding epitope X′ in a corresponding target molecule (ortholog) Y′ in animal species used for preclinical studies, but X′ may be different from X present in the corresponding target molecule in humans. Therefore, oftentimes, the monoclonal antibody cannot specifically recognize the ortholog and bind to the molecule. Even among groups of monoclonal antibodies that have reactivity to human and primate antigens, there are many examples of antibodies that only react with human and chimpanzee antigen homologs. For example, such cases have been observed for anti-CD3 monoclonal antibodies. One of the most widely used CD3 complexes-specific monoclonal antibodies that has the most properties determined is OKT-3. OKT-3 reacts with chimpanzee CD3 but does not react with CD3 homologs of other primates such as rhesus monkeys or canine CD3 (Non-Patent Document 2). On the other hand, there are examples of monoclonal antibodies that recognize rhesus antigens but not their human orthologs. An example in this group is FN-18, which is a monoclonal antibody against rhesus monkey-derived CD3 (Non-patent Document 2).
Several strategies have been adopted to counter problems with preclinical animal studies caused by the high specificity of such monoclonal antibodies.
The first known approach is to perform preclinical studies on test antibody pharmaceuticals using a chimpanzee model. Chimpanzees are the closest genetic relative of humans, and since their genome has 99% identity to the human genome, variations of the target molecule specifically bound by the test antibody pharmaceutical in chimpanzees are highly likely to be identical to the variations of this molecule in humans. In fact, Schlereth et al. have discovered that the variations in CD3 are common between humans and chimpanzees (Non-patent Document 3). Therefore, the risk that this molecule will not be recognized by the test antibody pharmaceutical in chimpanzees is considered to be low. However, studies using chimpanzees are very costly, and have ethical problems as well. Furthermore, since chimpanzees are animals in danger of extinction and the number of animals that can be used in experiments is severely limited, such preclinical studies on chimpanzees are excluded from the development of most test antibody pharmaceuticals.
The second approach is the approach of adapting the molecule used in preclinical studies to the animal used in the studies. In this approach, essential safety information is obtained in preclinical studies by constructing a so-called “surrogate” antibody for administration to test animals. Generally, such a surrogate antibody is an antibody that specifically recognizes a test-animal ortholog of the target molecule bound by the non-surrogate antibody (the actual test antibody pharmaceutical for humans), and is an antibody that has been modified to bind to the ortholog. Therefore, in the approach using such a “surrogate” antibody, one must individually develop two different molecules: the clinical test pharmaceutical agent, and a preclinical test pharmaceutical agent to be used in the preclinical studies on animal species, which has target specificity corresponding to the clinical pharmaceutical agent, and of which safety and such must be examined. The great disadvantage of such a surrogate approach is that the surrogate antibody for the preclinical studies is a modified product of the clinical test antibody pharmaceutical. Therefore, data obtained preclinical studies using a surrogate antibody may not often be directly applicable to humans. Therefore, the predictability of clinical stud results based on preclinical study results using these approaches may decrease.
The above-mentioned approach adapts the test pharmaceutical agent so that it is suitable for the animal used in preclinical studies. On the other hand, other known approaches adapt animals used in the preclinical studies to the candidate pharmaceutical agent to be administered to humans.
An example of adapting a test animal to a test antibody pharmaceutical intended for administration to humans is producing a transgenic animal that expresses the human molecule to which the test antibody pharmaceutical specifically binds instead of the non-human molecule intrinsic to the test animal species. In this method, the test antibody pharmaceutical administered in preclinical studies is expected to bind to a human antigen in the transgenic test animal. For example, in a study conducted by Bugelski et al., preclinical safety evaluation was performed on the monoclonal antibody keliximab using human CD4 transgenic mice to predict long-term treatment of rheumatoid arthritis in human patients. Keliximab is a monoclonal antibody that has specificity to CD4 of humans and chimpanzees. Bugelski et al. conclude that the use of a human protein-expressing transgenic mouse provides a useful alternative method to studies conducted in chimpanzees using biological pharmaceuticals that have limited cross-species specificity. However, production of transgenic animals for test purposes is time consuming and costly since it demands a to of work.